When installing a Fantastic Fan on my pal's fiberglass
bodied motorhome an interior trim ring was needed so
I spoke to the manufacturer and asked about the foam
gasket that came with the kit. The representative
told me that the gasket was specially made and
silicone impregnated specially for this application.
The gasket worked well and the vent has not leaked.
Of course Dicor was puddled on all the screw heads
and around the edge of the vent.
On my large 5th wheel trailer I installed a similar fan
and used butyl tape between the fan frame and the
elastomeric membrane that covers the roof. Again,
Dicor was used as the final step. No leaks
Note on using butyl tape: Make sure it is warm and
very pliable. Sometimes I use a double layer so that
there is plenty of 'squeeze-out'. After letting the
installation sit over night trim off the excess and
then use Dicor.
Hope this helps. Larry H